Gastrointestinal fat digestion consists of three steps, (i) the dispersion of bulk fat into emulsion droplets, (ii) the enzymatic hydrolysis of fat at the emulsion-water interface and (iii) the desorption and dispersion of digested fats, which are still water-insoluble, into smaller particles of micelles suitable for fat absorption (Carey, M. C., Small, D. M., and Bliss, C. M. 1983. Ann. Rev. Physiol. 45, 651-677). The intestinal content during fat digestion and absorption consists therefore of two phases, an emulsified oil phase and a micellar phase (Hoffmann, A. F., and Small, D. M. 1967. Ann. Rev. Med. 18, 333-376).
Convincing evidence now exists that fat is chiefly absorbed by the BBM from the micellar phase containing bile salts, 2-monoacyl glycerols, fatty acids, cholesterol and phospholipids or their lyso derivatives (Borgstrom, 1962. Gastroenterology, 43,216-219; Senior, J. R. 1964. J. Lipid Res. 4, 109-130).
it is well-known that cholesterol and other lipids are most efficiently absorbed by the BBM in the presence of bile salts (Borgstrom, B. 1974. Biomembranes (Smyth, D. H., Ed.) 4B, 555-620, Plenum Press, New York; Thomson, A. B. R., and Dietschy, J. M. 1981. Physiology of the Gastrointestinal Tract (Johnson, L. R., Ed.) 1147-1220, Raven Press, New York; Thurnhofer, H. and Hauser, H. 1990. Biochim. Biophys. Acta 1024, 249-262). Bile diversion reduces the uptake of not only cholesterol, but also of other lipids such as 2-monoacylglycerols, fatty acids and phospholipids. There are several ways in which bile salt are considered to facilitate fat absorption as was pointed out before (for example, Thomson and Dietschy, J. M. 1981. Physiology of the Gastrointestinal Tract (Johnson, L. R., Ed.) 1147-1220, Raven Press, New York): (i) bile salt micelles solubilize the products of fat digestion and serve as a vehicle to bring these products close to the site of fat absorption. There is an equilibrium partitioning of the lipids between the micellar phase and the aqueous phase, and lipid monomers diffuse into the external lipid monolayer of the BBM. (ii) Bile salt micelles adsorb or bind to the luminal surface of the BBM and lipid molecules are either enzymatically or by diffusion incorporated into the BBM. (iii) Small bile salt micelles containing the products of fat digestion are incorporated as a whole into the BBM in a process akin to pinocytosis.
Previous studies by Wilson and Treanor (Wilson, F. A. and Treanor, L. L. 1977. J. Membrane Biol. 33, 213-230) concluded that the interaction of .sup.14 C-labeled taurodeoxycholate with BBM from rat small intestines is unspecific and independent of membrane proteins. These studies reflected the interaction of the bile salt with the lipid bilayer of the BBM. Their conclusions are based on the finding that boiling of the suspension of brush border membrane vesicles (BBMV) and proteolysis of BBM with trypsin had no effect on the extent of binding of the bile salt.
Klip et al. (Klip, A., Grinstein, S., and Semenza, G. 1979. J. Membrane Biol. 51, 47-73) showed by sodium dodecyl-sulfate polyacrylamide gel electrophoresis that not only lipids are extracted from BBMV in the presence of excess bile salt micelles but also proteins. Whether these proteins are liberated from the membrane by the activity of intrinsic proteinases of the BBM or whether integral membrane proteins are distributed into the micellar phase is not clear from the data of Klip et al.
U.S. Pat. No. 4,806,532 to Dousa discloses a method of inhibition of transport of phosphate across the epithelial cell membrane by contacting the epithelial cells with an effective amount of photophonoformic acid (PFA), for example. PFA is an inhibitor of sodium phosphate co-transport across the luminal brush border membrane of the renal proximal tubulars. Dousa does not recognize the existence of a mediating protein in the bile salt and brush border membrane reaction. Dousa is concerned with inhibiting salt transport.
U.S. Pat. No. 4,760,135 to Diedrich et al., discloses the inhibition of uptake by the BBM, and is directed to restricting the uptake of glucose and sugar. Diedrich et al. do not recognize the existence of the protein.
Ikeda et al. (Ikeda et al. 1988. Journal of Lipid Research, 29, 1573-1591) studies several parameters of cholesterol absorption including the effect of .beta.-Sitosterol on micellar incorporation of cholesterol both in vitro and in vivo. Ikeda explains the discrepancy in the rate of absorption of cholesterol in vitro and in vivo by suggesting that cholesterol absorption proceeds through carrier mediated and/or energy dependant processes. Ikeda does not suggest what carrier mediated processes may be occurring.
Proulx et al. (Proulx et al. 1986. Experimental Biology, 45, 335-343) do not recognize the existence of a mediating protein but investigate the uptake of cholesterol by brush border membranes in the presence of a calcium cation. Proulx et al. are uncertain whether the effect of the calcium cation is to promote diffusion of cholesterol containing micelles with the brush border membrane, but suggests the calcium cation might promote micelle membrane interactions by masking negative charges at the reactant surfaces.
Tellier et al. (Tellier, C., Vallet-Strouve, C., Akoka, S., and Poignant, S. 1987. Eur. Biophys. J. 15, 177-184) investigates the relationship between bile salts and brush border membranes. Tellier et al. have discovered that taurocholate and other biliary salts produced considerable alteration in membranes except for brush border membranes. Tellier et al. have not investigated the reasons for this difference, but they do suggest that taurocholate interacts with brush border membrane bilayer. This publication teaches away from the existence of a protein mediating the reaction between bile salts and brush border membranes.